(1) A twisted-pair cable is used as the bus, and the nodes are connected in series. The length of the lead wire from the bus to each node should be as short as possible to minimize the influence of the reflected signal in the lead wire on the bus signal. Although some network connections are incorrect, they may still work normally at short distances and low speeds, but as the communication distance increases or the communication speed increases, the adverse effects will become more and more serious. The main reason is that the signal is reflected at the end of each branch. After being superimposed with the original signal, the signal quality will be degraded.
(2) Pay attention to the continuity of the characteristic impedance of the bus. Signal reflections will occur at the point where the impedance is discontinuous. The following situations are prone to produce this discontinuity: different sections of the bus use different cables, or there are too many transceivers installed close to each other on a certain section of the bus, and the excessively long branch line leads to the bus. In short, a single, continuous signal channel should be provided as a bus.
(3) Pay attention to the problem of terminal load resistance. The whole network can work well without adding terminal load resistance when the equipment is short and the distance is small, but the performance will decrease as the distance increases. Theoretically, when sampling at the midpoint of each received data signal, as long as the reflected signal attenuates sufficiently low at the beginning of sampling, matching can be ignored. But this is difficult to grasp in practice. An article from MAXIM in the United States mentioned an empirical principle that can be used to determine what data rate and cable length need to be matched: when the signal transition time (rise or fall time) When the electrical signal is more than 3 times the time required for one-way transmission of the electrical signal along the bus, no matching can be added.
Generally, terminal resistance is used for terminal matching. RS-485 should be connected in parallel at the beginning and end of the bus cable. The terminal resistance is 120Ω in the RS-485 network. The resistance equivalent to the characteristic impedance of the cable, because the characteristic impedance of most twisted-pair cables is about 100-120Ω. This kind of matching method is simple and effective, but it has a shortcoming. The matching resistor consumes a lot of power, which is not suitable for systems with stricter power consumption restrictions. Another power-saving matching method is RC matching. Using a capacitor C to block the DC component can save most of the power. However, the value of the capacitor C is a difficult point, and it is necessary to make a compromise between power consumption and matching quality. There is also a matching method using diodes. Although this scheme does not achieve true "matching", it uses the clamping effect of the diodes to quickly weaken the reflected signal and achieve the purpose of improving signal quality, with significant energy-saving effects.
